British people will forever associate Abu Musab al-Zarqawi, who was killed yesterday in his late 30s, with the kidnap and beheading in October 2004 of Ken Bigley. Zarqawi, the self-styled leader of "al-Qaida in the land of the two rivers", is also believed to have personally decapitated the 26-year-old American hostage Nick Berg earlier that year.

Yet increasingly his targets were Iraqis; the number of Shia civilians that his minions have slain since 2003 grotesquely eclipses the number of foreigners he is known to have dispatched. Zarqawi's antagonism towards Shias and their beliefs - evidenced in a broadcast a week ago - belied al-Qaida's claim to represent the interests of all Muslims.

Zarqawi also played a pivotal, if curious role, in the US decision to invade Iraq. Back in February 2003, secretary of state Colin Powell cited his presence in Baghdad as proof of ties between Saddam Hussein and al-Qaida - ties that "justified" military intervention. The claim proved increasingly suspect, but Zarqawi's reputation soared.

Some assert that Washington inflated Zarqawi's importance but the US was not alone in feeling threatened. On August 9 2003, Jordan named him as chief suspect in a suicide attack on its Baghdad embassy. His name was then associated with a deluge of atrocities. On August 19 Baghdad's UN headquarters was bombed, killing 22 people including the UN special envoy Sergio Viera de Mello. Soon afterwards a massive blast killed the Shia leader, Ayatollah Baqer al-Hakim (obituary, August 29 2003), and 83 worshippers outside the Imam Ali mosque in Najaf. Explosions on February 1 2004, during the Ashura festival, simultaneously slaughtered about 185 Shia celebrants in Karbala and Baghdad. Eleven days later, 102 Iraqi police recruits died in two car-bombings

A 17-page letter intercepted in January 2004, purportedly from Zarqawi to Osama bin Laden, demanded civil war between Iraq's Sunni minority and Shia majority. Initially, Iraqis dismissed Zarqawi as an foreign interloper without local support. However, he began attracting Sunnis, downhearted after Saddam's defeat, and bigots cheered when he called Shias "a sect of treachery and betrayal ... the lurking snake, the crafty and malicious scorpion".

In March 2004, Zarqawi's Jordanian, Yemeni, Saudi and Sudanese fighters merged with an indigenous Sunni Islamist group, Salafiya al-Mujahedia. There were rumours of more cynical unions with former Baathists and kidnappers. News of the US abuses at Abu Ghraib fuelled the flames. Yet the nationalist resistance in Iraq was nauseated by Zarqawi's gruesome TV beheading of Berg. In Lebanon, the Hizbullah condemned his contempt for civilian lives. The Salvation Front ordered him to "leave Iraq or die". The Muslim Council of Britain, whose officials tried but failed to free Bigley, castigated his actions as anathema to Islam.

None of this shamed Zarqawi. Two anti-kidnapping Sunni sheikhs were murdered in September 2004. The CIA believes that he personally sawed off the heads of two kidnapped American contractors. His followers boasted of killing 35 children in Baghdad on September 30 2004. Long before Zarqawi's Iraqi escapades, Jordanian agents had linked him to plots in Amman and Germany.

Despite this worldwide intelligence interest, Zarqawi's identity remains a mystery. He was probably born in Zarqa, an industrial town east of Jordan's capital, Amman. "Zarqawi" merely denoted his birthplace; his true name was likely Ahmad Fadil Nazal al-Khalayleh. Rather than being Palestinian - as was once believed - he probably hailed from the Bani Hassan, a Jordanian desert tribe.

He grew up in poverty, dropped out of school and took to heavy drinking, tattoos and fighting. Allegedly jailed for sexual assault, he embraced militant Islam and travelled in Afghanistan in 1989. With the Soviets already defeated, he was left to edit a magazine for demobilised mujaheddin. In Afghanistan he met bin Laden, and his penchant for expertly synchronised suicide bombings recalled al-Qaida's methods. Yet colleagues said relations between the two men cooled. By 2000 Zarqawi was running training camps for his Jund al-Sham (Levantine Soldiers) in Herat, hundreds of miles from bin Laden's bases.

In 1992 Zarqawi joined Bayat al-Imam (Loyalty to the Imam), a radical Jordanian clique led by another former convict, Issam Barqawi. Bayat favoured a return to what they see as the wellsprings of Islam; it damned the Jordanian monarchy, attacked democracy and justified the mass murder of Jews and Christians.

Reportedly jailed for possessing arms in 1993, Zarqawi became a taciturn loner; fellow prisoners recalled him intimidating inmates with a mere glare. When not studying the Qur'an, he would lift weights to beef up his slight frame. Released in an amnesty in 1999, he was soon to be sentenced in absentia for plotting to bomb tourists celebrating the new millennium at the Radisson SAS hotel in Amman. Bayat also allegedly planned to kill Christians pilgrims.

In 2000, Zarqawi escaped with his ailing mother to Peshawar in Pakistan. Late the following year, he was apparently wounded during the US bombardment of al-Qaida's Afghan bases. Surgeons at Baghdad's Ibn Sina hospital operated on his leg in May 2002. Intelligence agents then said he was building poison factories for an al-Qaida linked Kurdish group, Ansar al-Islam, in north-east Iraq. American and Kurdish forces destroyed Ansar camps in April 2003, but Zarqawi slipped away. Two months later, Washington charged Iran with harbouring him.

Soon Zarqawi was back in Iraq, where he revived his group, Tawhid al-Jihad (Monotheism and Holy War, or Unity and Struggle). Its epicentre was Falluja, where four US private security agents were killed in April 2004. The American response was a three-week assault on the city.

In February 2004 Washington raised Zarqawi's bounty to $25m - the same as it had put on Saddam and bin Laden. US Task Force 626 pledged to capture or eliminate him. Iraqi police caught a Zarqawi lieutenant, Omar Baziyani, on June 4; American raids then killed about 100 Tawhid followers in Falluja; and on September 17 a US missile slew Zarqawi's ideological mentor, Sheikh Abu Anas al-Shami, in Baghdad.

Some detected the outlaw's hand outside Iraq. Spanish officials investigated his possible role in the train bombings that killed 191 in Madrid on March 11 2004. Zarqawi was certainly growing more audacious. He took credit for car bombs that killed 60 in Najaf and Karbala in December 2004; rocket attacks on Israel and US navy vessels in Jordan the following August, and a triple attack on three Amman hotels which killed another 60 on November 9 2005. Unprecedented demonstrations against terror ensued by ordinary Sunni Muslims.

Zarqawi continued, despite the killing and capture of many of his aides. Most significantly - and curiously, in the light of his growing unpopularity - bin Laden accepted his oath of allegiance as recently as December 2004, in effect making him an al-Qaida brand franchise. In May last year he justified the collateral killing of Muslims, where necessary. The following month his website launched a slick 46-minute video, All Religion Will be for Allah. The man whose remoteness earned him the nickname al-Gharib (the Stranger) - a moniker he used to sign letters - was videoed that December cradling a gun, swathed in a suicide bomber's vest, and, for the first time, unmasked. It was another stake at self-promotion.

Yet he failed to stop last yer's Iraqi elections, despite his eve-of-poll tirade against democracy, "the very essence of heresy, polytheism and error". Ultimately, his brutality tarnished any aura, offered little but nihilism and repelled Muslims worldwide. Two wives and four children survive him.